Help Desk SLA (Service level Agreement) is an agreement between company and its customers, A SLA rule defines response time and resolution for a ticket. Response time decides how fast staff should respond to customers request, where as Resolution time decides the tickets final resolution.
SLA rules acts as a performance-meter for staff agents. You can draw a report that shows how your staff agents are responding and resolving given set of tickets. SLA helps staff agents to respond to the tickets in a timely manner.